Systems and methods for two-level protection of power conversion systems

ABSTRACT

Systems and methods are provided for protecting a power conversion system. A system controller includes a two-level protection component and a driving component. The two-level protection component is configured to detect an output power of a power conversion system and generate a protection signal based on at least information associated with the output power. The driving component is configured to generate a drive signal based on at least information associated with the protection signal and output the drive signal to a switch associated with a primary current flowing through a primary winding of the power conversion system. The driving component is further configured to generate the drive signal corresponding to a first switching frequency to generate the output power equal to a first power threshold and generate the drive signal corresponding to a second switching frequency to generate the output power equal to a second power threshold.

1. CROSS-REFERENCES TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.15/220,223, filed Jul. 26, 2016, which is a continuation of U.S. patentapplication Ser. No. 13/948,080, filed Jul. 22, 2013, which claimspriority to Chinese Patent Application No. 201310228620.6, filed Jun. 8,2013, all of the above-referenced applications being commonly assignedand incorporated by reference herein for all purposes.

2. BACKGROUND OF THE INVENTION

The present invention is directed to integrated circuits. Moreparticularly, the invention provides a system and method for two-levelprotection. Merely by way of example, the invention has been applied toa power conversion system. But it would be recognized that the inventionhas a much broader range of applicability.

Generally, a power conversion system includes one or more mechanisms forover-current protection so that the system can be protected against anexcessive current if the output load exceeds a limit. For example, if ahigh output capability is needed for the power conversion system duringa short period of time to drive a load (e.g., a printing system, acapacitive load) and/or for over-clocking of a central processing unit,a current threshold for over-current protection may need to be set at ahigh magnitude in order to provide such high output capability. But whenthe power conversion system provides the high output capability, thesystem current may become so high as to cause over-saturation of thesystem, even though the system current has not yet reached the highcurrent threshold and the over-current protection is not yet triggered.Then, the power conversion system may become over-heated and thus fail.

Hence it is highly desirable to improve the techniques of protectingpower conversion systems.

3. BRIEF SUMMARY OF THE INVENTION

The present invention is directed to integrated circuits. Moreparticularly, the invention provides a system and method for two-levelprotection. Merely by way of example, the invention has been applied toa power conversion system. But it would be recognized that the inventionhas a much broader range of applicability.

According to one embodiment, a system controller for protecting a powerconversion system includes a two-level protection component and adriving component. The two-level protection component is configured todetect an output power of a power conversion system and generate aprotection signal based on at least information associated with theoutput power of the power conversion system. The driving component isconfigured to generate a drive signal based on at least informationassociated with the protection signal and output the drive signal to aswitch associated with a primary current flowing through a primarywinding of the power conversion system. The driving component is furtherconfigured to, generate the drive signal corresponding to a firstswitching frequency to generate the output power equal to a first powerthreshold, and generate the drive signal corresponding to a secondswitching frequency to generate the output power equal to a second powerthreshold, the second power threshold being larger than the first powerthreshold, the second switching frequency being larger than the firstswitching frequency. The two-level protection component is furtherconfigured to, in response to the output power being equal to or largerthan the first power threshold for a first predetermined period of time,generate the protection signal to cause the switch to open and remainopen to protect the power conversion system, and in response to theoutput power being equal to or larger than the second power thresholdfor a second predetermined period of time, generate the protectionsignal to cause the switch to open and remain open to protect the powerconversion system, the second predetermined period of time being shorterthan the first predetermined period of time.

According to another embodiment, a system controller for regulating apower conversion system includes a modulation component and a drivingcomponent. The modulation component is configured to receive a feedbacksignal associated with an output signal of a power conversion system andgenerate a modulation signal based on at least information associatedwith the feedback signal, the modulation signal being associated with amodulation frequency. The driving component is configured to generate adrive signal based on at least information associated with themodulation signal and output the drive signal to a switch associatedwith a primary current flowing through a primary winding of the powerconversion system, the drive signal being associated with the modulationfrequency. The modulation component is further configured to, increasethe modulation frequency from a first frequency magnitude to a secondfrequency magnitude in response to the feedback signal increasing from afirst feedback magnitude to a second feedback magnitude, keep themodulation frequency at the second frequency magnitude if the feedbacksignal is larger than the second feedback magnitude and smaller than thethird feedback magnitude, and increase the modulation frequency from thesecond frequency magnitude to a third frequency magnitude in response tothe feedback signal increasing from the third feedback magnitude to afourth feedback magnitude.

In one embodiment, a method for protecting a power conversion systemincludes, detecting an output power of a power conversion system,generating a protection signal based on at least information associatedwith the output power of the power conversion system, and receiving theprotection signal. The method further includes, processing informationassociated with the protection signal, generating a drive signal basedon at least information associated with the protection signal, andoutputting the drive signal to a switch associated with a primarycurrent flowing through a primary winding of the power conversionsystem. The generating a drive signal based on at least informationassociated with the protection signal includes, generating the drivesignal corresponding to a first switching frequency to generate theoutput power equal to a first power threshold, and generating the drivesignal corresponding to a second switching frequency to generate theoutput power equal to a second power threshold, the second powerthreshold being larger than the first power threshold, the secondswitching frequency being larger than the first switching frequency. Thegenerating a protection signal based on at least information associatedwith the output power of the power conversion system includes, inresponse to the output power being equal to or larger than the firstpower threshold for a first predetermined period of time, generating theprotection signal to cause the switch to open and remain open to protectthe power conversion system, and in response to the output power beingequal to or larger than the second power threshold for a secondpredetermined period of time, generating the protection signal to causethe switch to open and remain open to protect the power conversionsystem, the second predetermined period of time being shorter than thefirst predetermined period of time.

In another embodiment, a method for regulating a power conversion systemincludes, receiving a feedback signal associated with an output signalof a power conversion system, processing information associated with thefeedback signal, and generating a modulation signal based on at leastinformation associated with the feedback signal, the modulation signalbeing associated with a modulation frequency. The method furtherincludes, receiving the modulation signal, processing informationassociated with the modulation signal, generating a drive signal basedon at least information associated with the modulation signal, andoutputting the drive signal to a switch associated with a primarycurrent flowing through a primary winding of the power conversionsystem, the drive signal being related to the modulation frequency. Thegenerating a modulation signal based on at least information associatedwith the feedback signal includes, increasing the modulation frequencyfrom a first frequency magnitude to a second frequency magnitude inresponse to the feedback signal increasing from a first feedbackmagnitude to a second feedback magnitude, keeping the modulationfrequency at the second frequency magnitude if the feedback signal islarger than the second feedback magnitude and smaller than the thirdfeedback magnitude, and increasing the modulation frequency from thesecond frequency magnitude to a third frequency magnitude in response tothe feedback signal increasing from the third feedback magnitude to afourth feedback magnitude.

Depending upon embodiment, one or more benefits may be achieved. Thesebenefits and various additional objects, features and advantages of thepresent invention can be fully appreciated with reference to thedetailed description and accompanying drawings that follow.

4. BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a simplified diagram for a power conversion system withtwo-level protection according to an embodiment of the presentinvention.

FIG. 2 is a simplified timing diagram for the output power of the powerconversion system as shown in FIG. 1 according to an embodiment of thepresent invention.

FIG. 3(a) is a simplified diagram for certain components of thecontroller as part of the power conversion system as shown in FIG. 1according to an embodiment of the present invention.

FIG. 3(b) is a simplified diagram for certain components of thecontroller as part of the power conversion system as shown in FIG. 1according to another embodiment of the present invention.

FIGS. 4(a) and 4(b) are simplified diagrams showing the peak value ofthe current-sensing signal and the modulation frequency of the powerconversion system as shown in FIG. 1 as functions of the feedback signalrespectively according to certain embodiments of the present invention.

5. DETAILED DESCRIPTION OF THE INVENTION

The present invention is directed to integrated circuits. Moreparticularly, the invention provides a system and method for two-levelprotection. Merely by way of example, the invention has been applied toa power conversion system. But it would be recognized that the inventionhas a much broader range of applicability.

FIG. 1 is a simplified diagram for a power conversion system withtwo-level protection according to an embodiment of the presentinvention. This diagram is merely an example, which should not undulylimit the scope of the claims. One of ordinary skill in the art wouldrecognize many variations, alternatives, and modifications. The powerconversion system 100 includes a controller 102, a switch 104, a primarywinding 106, a secondary winding 108, an isolated feedback component110, resistors 112 and 114, a diode 116, and a capacitor 118. Thecontroller 102 includes a modulation component 120, a logic controller122, a driving component 124, a leading-edge-blanking (LEB) component128, and a two-level protection component 126. In addition, thecontroller 102 includes terminals 130, 132, 134, 136, and 138. Forexample, the two-level protection component 126 is a two-levelover-power protection component. In another example, the switch 104 is atransistor. In yet another example, the switch 104 is a field effecttransistor. In yet another example, the switch 104 is a bipolartransistor. In some embodiments, the LEB component 128 is omitted.

According to one embodiment, the two-level protection component 126receives a feedback signal 140 associated with an output signal 198 anda current-sensing signal 142 associated with a primary current 144flowing through the primary winding 106, and outputs a control signal146 to the logic controller 122. For example, if the output power isbelow a first-level threshold, the power conversion system 100 performsnormal operations. In yet another example, if the output power becomesequal to or larger than the first-level threshold and also remains equalto or larger than the first-level threshold for at least a firstpredetermined period of time, the two-level protection component 126changes the signal 146 in order to shut down the power conversion system100. In yet another example, if the output power becomes equal to orlarger than the second-level threshold and also remains equal to orlarger than a second predetermined period of time, the two-levelprotection component 126 changes the signal 146 in order to shut downthe power conversion system 100. In yet another example, if the outputpower increases from the first-level threshold to the second-levelthreshold, the two-level protection component 126 outputs the signal 146to the logic controller 122 in order to increase the modulationfrequency of the power conversion system 100. For example, if the powerconversion system 100 is shut down, the switch 104 is kept open.

In yet another example, after being shut down, the system 100 restarts(e.g., automatically or manually) and starts modulation again. In yetanother example, when the system 100 starts modulation again, the switch104 is closed (e.g., being turned on) and opened (e.g., being turnedoff) at a modulation frequency.

According to another embodiment, the modulation component 120 receivesthe feedback signal 140 and outputs a signal 150 to the logic controller122. For example, the logic controller 122 outputs a signal 148 to thedriving component 124 which outputs a drive signal 196 to affect thestatus of the switch 104. In another example, the feedback signal 140 isgenerated by the isolated feedback component 110 based on at leastinformation associated with an output signal 198 (e.g., V_(out)). In yetanother example, the signal 148 and the signal 196 are modulationsignals and associated with a modulation frequency.

As discussed above and further emphasized here, FIG. 1 is merely anexample, which should not unduly limit the scope of the claims. One ofordinary skill in the art would recognize many variations, alternatives,and modifications. In one embodiment, the two-level protection component126 does not receive the feedback signal 140. In another embodiment, oneor more components are configured to process the feedback signal 140 andgenerate a processed signal. For example, the processed signal isreceived by the modulation component 120. In another example, theprocessed signal is received by the modulation component 120 and thetwo-level protection component 126.

FIG. 2 is a simplified timing diagram for the output power of the powerconversion system 100 according to an embodiment of the presentinvention. This diagram is merely an example, which should not undulylimit the scope of the claims. One of ordinary skill in the art wouldrecognize many variations, alternatives, and modifications. The waveform202 represents the output power of the power conversion system 100 as afunction of time, and the waveform 204 represents the drive signal 196of the power conversion system 100 as a function of time. For example,t₀≤t₁≤t₂≤t₃≤t₄≤t₅≤t₇≤t₈≤t₉≤t₁₀≤t₁₁≤t₁₂≤t₁₃≤t₁₄.

According to one embodiment, during a time period between the time t₀and the time t₁, the output power of the power conversion system 100 isbelow a first power threshold 206 (e.g., as shown by the waveform 202),and a first protection timer associated with the first level protectionis not activated. For example, the output power of the power conversionsystem 100 is below a second power threshold 208, and a secondprotection timer associated with the second level protection is notactivated. In another example, the drive signal 196 changes between alogic high level and a logic low level at a modulation frequency (e.g.,as shown by the waveform 204), and in response the switch 104 is closedand opened within a switching period corresponding to the modulationfrequency.

According to another embodiment, at the time t₁, the output power of thepower conversion system 100 becomes equal to or larger than the firstpower threshold 206 (e.g., as shown by the waveform 202), and the firstprotection timer is activated to count time. For example, the outputpower of the power conversion system 100 is below a second powerthreshold 208, and a second protection timer is not activated.

According to yet another embodiment, during the time period between thetime t₁ and the time t₂, the output power of the power conversion system100 remains equal to or larger than the first power threshold 206 (e.g.,as shown by the waveform 202), and the first protection timer continuesto count time. For example, the modulation frequency increases. Inanother example, at the time t₂, the output power of the powerconversion system 100 becomes equal to or larger than the second powerthreshold 208 (e.g., as shown by the waveform 202), and the secondprotection timer is activated to count time. In yet another example, thesecond power threshold 208 is larger than the first power threshold 206.

In one embodiment, during the time period between the time t₂ and thetime t₃, the output power of the system 100 remains equal to or largerthan the first power threshold 206 (e.g., as shown by the waveform 202),and the first protection timer continues to count time. For example, theoutput power of the system 100 remains equal to or larger than thesecond power threshold 208 and the second protection timer continues tocount time, during the time period between the time t₂ and the time t₃.In another example, at the time t₃, the output power of the powerconversion system 100 becomes smaller than the second power threshold208 (e.g., as shown by the waveform 202). In yet another example, at thetime t₃, the second protection timer stops counting, but does not causethe power conversion system 100 to shut down because the time periodbetween the time t₂ and the time t₃ is smaller than the secondpredetermined time period (e.g., T₂) in duration. In yet anotherexample, at the time t₃, the second protection timer is cleared. In yetanother example, the second predetermined time period (e.g., T₂) issmaller in duration than the first predetermined time period (e.g., T₁).

In another embodiment, during the time period between the time t₃ andthe time t₄, the output power of the power conversion system 100 remainsequal to or larger than the first power threshold 206 (e.g., as shown bythe waveform 202), and the first protection timer continues to counttime. For example, from the time t₁ to the time t₄, the output power ofthe power conversion system 100 remains equal to or larger than thefirst power threshold 206, and the time interval between the time t₁ tothe time t₄ is equal to or larger than the first predetermined timeperiod (e.g., T₁); in response, the first protection timer causes thepower conversion system 100 to shut down at the time t₄. In anotherexample, at the time t₄, the first protection timer stops counting andis cleared.

After being shut down, the power conversion system 100 restarts at thetime t₅ (e.g., automatically or manually) and starts modulation again,according to some embodiments. For example, when the system 100 startsmodulation, the drive signal 196 begins to change between the logic highlevel and the logic low level again at a particular modulation frequency(e.g., as shown by the waveform 204), and in response the switch 104 isclosed and opened within a switching period corresponding to theparticular modulation frequency. In another example, the output power ofthe system 100 remains smaller than the first power threshold 206 duringthe time period between t₅ and t₆, and the first protection timer is notactivated. In yet another example, the output power of the system 100remains smaller than the second power threshold 208 during the timeperiod between t₅ and t₆, and the second protection timer is notactivated.

According to one embodiment, at the time t₆, the output power of thepower conversion system 100 becomes equal to or larger than the firstpower threshold 206, and the first protection timer is activated tocount time. For example, the output power of the power conversion system100 is below the second power threshold 208, and the second protectiontimer is not activated.

According to another embodiment, during the time period between the timet₆ and the time t₇, the output power of the power conversion system 100remains equal to or larger than the first power threshold 206 (e.g., asshown by the waveform 202), and the first protection timer continues tocount time. For example, the modulation frequency increases. In anotherexample, at the time t₇, the output power of the power conversion system100 becomes equal to or larger than the second power threshold 208(e.g., as shown by the waveform 202), and the second protection timer isactivated to count time.

According to yet another embodiment, during the time period between thetime t₇ and the time t₈, the output power of the system 100 remainsequal to or larger than the first power threshold 206 (e.g., as shown bythe waveform 202), and the first protection timer continues to counttime. For example, the output power of the system 100 remains equal toor larger than the second power threshold 208 and the second protectiontimer continues to count time, during the time period between the timet₇ and the time t₈. In another example, at the time t₈, the output powerof the power conversion system 100 becomes smaller than the second powerthreshold 208 (e.g., as shown by the waveform 202). In yet anotherexample, at the time t₈, the second protection timer stops counting, butdoes not cause the power conversion system 100 to shut down because thetime period between the time t₇ and the time t₈ is smaller than thesecond predetermined time period (e.g., T₂) in duration. In yet anotherexample, at the time t₈, the second protection timer is cleared.

According to yet another embodiment, during the time period between thetime t₈ and the time t₉, the output power of the system 100 remainsequal to or larger than the first power threshold 206 (e.g., as shown bythe waveform 202), and the first protection timer continues to counttime. For example, the output power of the system 100 is smaller thanthe second power threshold 208, and the second protection timer is notactivated to count time. In another example, at the time t₉, the outputpower of the power conversion system 100 becomes equal to or larger thanthe second power threshold 208 (e.g., as shown by the waveform 202), andthe second protection timer is activated again to count time.

In one embodiment, during the time period between the time t₉ and thetime t₁₀, the output power of the system 100 remains equal to or largerthan the first power threshold 206 (e.g., as shown by the waveform 202),and the first protection timer continues to count time. For example, theoutput power of the system 100 remains equal to or larger than thesecond power threshold 208 and the second protection timer continues tocount time, during the time period between the time t₉ and the time t₁₀.In another example, at the time t₁₀, the output power of the powerconversion system 100 becomes smaller than the second power threshold208 (e.g., as shown by the waveform 202). In yet another example, at thetime t₁₀, the second protection timer stops counting, but does not causethe power conversion system 100 to shut down because the time periodbetween the time t₉ and the time t₁₀ is smaller than the secondpredetermined time period (e.g., T₂) in duration. In yet anotherexample, at the time t₁₀, the second protection timer is cleared.

In another embodiment, during the time period between the time t₁₀ andthe time t₁₁, the output power of the power conversion system 100remains equal to or larger than the first power threshold 206 (e.g., asshown by the waveform 202), and the first protection timer continues tocount time. For example, at the time t₁₁, the output power of the powerconversion system 100 becomes smaller than the first power threshold206. In another example, at the time t₁₁, the first protection timerstops counting, but does not cause the power conversion system 100 toshut down because the time period between the time t₆ and the time t₁₁is smaller than the first predetermined time period (e.g., T₁) induration. In yet another example, at the time t₁₁, the first protectiontimer is cleared.

In yet another embodiment, during the time period between the time t₁₁and the time t₁₂, the output power of the power conversion system 100remains smaller than the first power threshold 206 (e.g., as shown bythe waveform 202), and the first protection timer is not activated. Forexample, the output power of the power conversion system 100 remainssmaller than the second power threshold 208 (e.g., as shown by thewaveform 202), and the second protection timer is not activated.

According to one embodiment, at the time t₁₂, the output power of thepower conversion system 100 becomes equal to or larger than the firstpower threshold 206 (e.g., as shown by the waveform 202), and the firstprotection timer is activated to count time. For example, the outputpower of the power conversion system 100 is below a second powerthreshold 208, and the second protection timer is not activated.

According to another embodiment, during the time period between the timet₁₂ and the time t₁₃, the output power of the power conversion system100 remains equal to or larger than the first power threshold 206 (e.g.,as shown by the waveform 202), and the first protection timer continuesto count time. For example, the modulation frequency increases. Inanother example, at the time t₁₃, the output power of the powerconversion system 100 becomes equal to or larger than the second powerthreshold 208 (e.g., as shown by the waveform 202), and the secondprotection timer is activated to count time.

According to yet another embodiment, during the time period between thetime t₁₃ and the time t₁₄, the output power of the system 100 remainsequal to or larger than the first power threshold 206 (e.g., as shown bythe waveform 202), and the first protection timer continues to counttime. For example, from the time t₁₃ to the time t₁₄, the output powerof the power conversion system 100 remains equal to or larger than thesecond power threshold 206, and the time interval between the time t₁₃to the time t₁₄ is equal to or larger than the second predetermined timeperiod (e.g., T₂); in response, the second protection timer causes thepower conversion system 100 to shut down at the time t₁₄. In yet anotherexample, at the time t₁₄, the second protection timer stops counting andis cleared. In yet another example, at the time t₁₄, the firstprotection timer stops counting and is cleared.

As discussed above and further emphasized here, FIG. 2 is merely anexample, which should not unduly limit the scope of the claims. One ofordinary skill in the art would recognize many variations, alternatives,and modifications. In one embodiment, if the output voltage of the powerconversion system 100 remains approximately constant, the simplifiedtiming diagram for the output power also approximates a timing diagramfor the output current of the power conversion system 100. In anotherembodiment, the two-level protection component 126 is a two-levelover-current protection component. For example, the two-levelover-current protection component performs a two-level over-currentprotection, so that the output current of the power conversion system100 is subject to two current thresholds and the output voltage 198 ofthe power conversion system 100 remains approximately constant.

FIG. 3(a) is a simplified diagram for certain components of thecontroller 102 as part of the power conversion system 100 according toan embodiment of the present invention. This diagram is merely anexample, which should not unduly limit the scope of the claims. One ofordinary skill in the art would recognize many variations, alternatives,and modifications. The controller 102 further includes an under-voltagelockout component 302, a reference signal generator 304, and a diode306. The modulation component 120 includes a signal processor 316, anoscillator 318, a slope-compensation component 320, comparators 326 and328, and an OR gate 330. The logic controller 122 includes a flip-flopcomponent 350, NOT gates 340 and 348, NOR gates 344 and 346, and an ANDgate 352. The two-level protection component 126 includes comparators322 and 324, an OR gate 342, flip-flop components 332 and 334, andcounter components 336 and 338. For example, resistors 308, 312 and 314,and a diode 310 are included in the modulation component 120. In anotherexample, the resistors 308, 312 and 314, and the diode 310 are includedin the two-level protection component 126. In yet another example, asingle comparator serves as both the comparator 324 and the comparator326.

According to one embodiment, the two-level protection component 126receives the feedback signal 140 and the current-sensing signal 142 inorder to determine when to trigger a first-level protection and asecond-level protection. For example, the signal processor 316 receivesa signal 354 associated with the feedback signal 140 and outputs aprocessed signal 356 to the oscillator 318 which outputs a signal 358 tothe slope-compensation component 320. In another example, theslope-compensation component 320 also receives the signal 354 andoutputs a signal 360 (e.g., V_(th_pwm)) to the comparator 328 whichgenerates a first comparison signal 374. In yet another example, thecomparator 322 compares the current-sensing signal 142 with a firstthreshold signal 362 (e.g., V_(p_1)), and outputs a second comparisonsignal 368 to the flip-flop component 332 which also receives the signal148 (e.g., PWM) and a signal 366 (e.g., PG) generated by the component304. In yet another example, the comparator 324 compares thecurrent-sensing signal 142 with a second threshold signal 364 (e.g.,V_(p_2)), and outputs a third comparison signal 370 to the flip-flopcomponent 334 which also receives the signal 148 (e.g., PWM) and thesignal 366 (e.g., PG). In yet another example, the comparator 326compares the current-sensing signal 142 with a second threshold signal364, and outputs a fourth comparison signal 372 to the OR gate 330. Inyet another example, the second threshold signal 364 (e.g., V_(p_2)) islarger than the first threshold signal 362 (e.g., V_(p_1)) in magnitude.In yet another example, the oscillator 318 outputs a signal 398 (e.g.,CLK) to the flip-flop component 350. In yet another example, the signal398 is associated with an oscillation frequency. In yet another example,the oscillation frequency is the same as the modulation frequencyassociated with the signal 148.

According to another embodiment, if the output load is light, thefirst-level protection or the second-level protection is not triggered.For example, the signal 360 (e.g., V_(th_pwm)) is determined accordingto the following equation:

$\begin{matrix}{V_{{th}\;\_\;{pwm}} = {{\frac{{FB} - V_{D}}{R_{1} + R_{2}} \times R_{1}} - {\Delta\; V_{slope}}}} & \left( {{Equation}\mspace{14mu} 1} \right)\end{matrix}$where FB represents the feedback signal 140, V_(D) represents a forwardvoltage of the diode 310, and R₁ represents the resistance of theresistor 314. In addition, R₂ represents the resistance of the resistor312, and ΔV_(slope) represents a parameter associated with the dutycycle of the signal 148 (e.g., a PWM signal). For example, the signal354 is determined according to the following equation:

$\begin{matrix}{{FB}_{div} = {\frac{{FB} - V_{D}}{R_{1} + R_{2}} \times R_{1}}} & \left( {{Equation}\mspace{14mu} 2} \right)\end{matrix}$where FB_(div) represents the signal 354. In another example, if thecurrent-sensing signal 142 becomes equal to or larger than the signal360 (e.g., V_(th_pwm)) in magnitude, the comparator 328 outputs thesignal 374 to change the signal 148 (e.g., PWM) in order to open (e.g.,turn off) the switch 104.

According to yet another embodiment, if the output load increases, thefeedback signal 140 increases in magnitude, and the signal 360 (e.g.,V_(th_pwm)) increases in magnitude. For example, if the signal 360(e.g., V_(th_pwm)) is equal to the first threshold signal 362 (e.g.,V_(p_1)) in magnitude and the current-sensing signal 142 becomes equalto or larger than the first threshold signal 362 (e.g., V_(p_1)), thecomparator 322 outputs the signal 368 (e.g., V₁) at a logic high levelin order to trigger the counter component 336 to count time. In anotherexample, the flip-flop component 332 samples the signal 368 upon afalling edge of the signal 148. In yet another example, if the signal368 is kept at the logic high level for a first predetermined timeperiod (e.g., T₁), the counter component 336 outputs a signal 376 (e.g.,at a logic high level) to trigger the first-level protection. In yetanother example, if the first-level protection is triggered, the powerconversion system 100 is shut down and the switch 104 keeps open. In yetanother example, after being shut down, the system 100 restarts (e.g.,automatically or manually) and starts modulation again.

According to yet another embodiment, if the output load continues toincrease, the feedback signal 140 continues to increase in magnitude,and the signal 360 (e.g., V_(th_pwm)) increases in magnitude. Forexample, if the signal 360 (e.g., V_(th_pwm)) becomes equal to or largerthan the second threshold signal 364 (e.g., V_(p_2)) in magnitude, thecomparator 324 outputs the signal 370 (e.g., V₂) at a logic high levelin order to trigger the counter component 338 to count time. In anotherexample, the flip-flop component 334 samples the signal 370 upon afalling edge of the signal 148.

In yet another example, if the signal 370 is kept at the logic highlevel for a second predetermined time period (e.g., T₂), the countercomponent 338 outputs a signal 378 (e.g., at a logic high level) totrigger the second-level protection. In yet another example, if thesecond-level protection is triggered, the power conversion system 100 isshut down and the switch 104 keeps open. In yet another example, afterbeing shut down, the system 100 restarts (e.g., automatically ormanually) and starts modulation again. In yet another example, thesecond predetermined time period (e.g., T₂) is smaller in duration thanthe first predetermined time period (e.g., T₁).

As discussed above and further emphasized here, FIG. 3(a) is merely anexample, which should not unduly limit the scope of the claims. One ofordinary skill in the art would recognize many variations, alternatives,and modifications. According to certain embodiments, the modulationcomponent 120 may include components not shown in FIG. 3(a). Accordingto some embodiments, one or more components included in the modulationcomponent 120, as shown in FIG. 3(a), may be omitted. According tocertain embodiments, the logic controller 122 may include components notshown in FIG. 3(a). According to some embodiments, one or morecomponents included in the logic controller 122, as shown in FIG. 3(a),may be omitted.

FIG. 3(b) is a simplified diagram for certain components of thecontroller 102 as part of the power conversion system 100 according toanother embodiment of the present invention. This diagram is merely anexample, which should not unduly limit the scope of the claims. One ofordinary skill in the art would recognize many variations, alternatives,and modifications. The controller 102 further includes an under-voltagelockout component 402, a reference signal generator 404, and a diode406. The modulation component 120 includes a signal processor 416, anoscillator 418, a slope-compensation component 420, comparators 426 and428, and an OR gate 430. The logic controller 122 includes a flip-flopcomponent 450, NOT gates 440 and 448, NOR gates 444 and 446, and an ANDgate 452. The two-level protection component 126 includes comparators422 and 424, an OR gate 442, flip-flop components 432 and 434, andcounter components 436 and 438. For example, resistors 408, 412 and 414,and a diode 410 are included in the modulation component 120. In anotherexample, the resistors 408, 412 and 414, and the diode 410 are includedin the two-level protection component 126.

For example, the under-voltage lockout component 402, the referencesignal generator 404, the diodes 406 and 410, the resistors 408, 412 and414, the signal processor 416, the oscillator 418, theslope-compensation component 420, the comparators 422, 426 and 428, theOR gates 430 and 442, the flip-flop components 432, 434 and 450, thecounter components 436 and 438, the NOT gates 440 and 448, the NOR gates444 and 446, and the AND gate 452 are the same as the under-voltagelockout component 302, the reference signal generator 304, the diodes306 and 310, the resistors 308, 312 and 314, the signal processor 316,the oscillator 318, the slope-compensation component 320, thecomparators 322, 326 and 328, the OR gates 330 and 342, the flip-flopcomponents 332, 334 and 350, the counter components 336 and 338, the NOTgates 340 and 348, the NOR gates 344 and 346, and the AND gate 352,respectively.

According to one embodiment, the two-level protection component 126receives the feedback signal 140 and the current-sensing signal 142 inorder to determine when to trigger protection. For example, the signalprocessor 416 receives a signal 454 (e.g., FB_div) associated with thefeedback signal 140 and outputs a processed signal 456 to the oscillator418 which outputs a signal 458 to the slope-compensation component 420.In another example, the slope-compensation component 420 also receivesthe signal 454 (e.g., FB_div) and outputs a signal 460 (e.g.,V_(th_pwm)) to the comparator 428 which generates a first comparisonsignal 474. In yet another example, the comparator 422 compares thecurrent-sensing signal 142 with a first threshold signal 462 (e.g.,V_(p_1)), and outputs a second comparison signal 468 to the flip-flopcomponent 432 which also receives the signal 148 (e.g., PWM) and asignal 466 (e.g., PG) generated by the component 404. In yet anotherexample, the comparator 424 compares the signal 454 (e.g., FB_div) witha threshold signal 494 (e.g., V_(th_pk)), and outputs a third comparisonsignal 470. In yet another example, the comparator 426 compares thecurrent-sensing signal 142 with a second threshold signal 464 (e.g.,V_(p_2)), and outputs a fourth comparison signal 472 to the OR gate 430.In yet another example, the signal 494 (e.g., V_(th_pk)) is larger thanthe first threshold signal 462 (e.g., V_(p_1)) in magnitude. In yetanother example, the oscillator 418 outputs a signal 498 (e.g., CLK) tothe flip-flop component 450. In yet another example, the signal 498 isassociated with an oscillation frequency. In yet another example, theoscillation frequency is the same as the modulation frequency associatedwith the signal 148.

According to another embodiment, if the output load is light, thefirst-level protection or the second-level protection is not triggered.For example, the signal 460 (e.g., V_(th_pwm)) is determined accordingto the following equation:

$\begin{matrix}{V_{{th}\;\_\;{pwm}} = {{\frac{{FB} - V_{D}}{R_{1} + R_{2}} \times R_{1}} - {\Delta\; V_{slope}}}} & \left( {{Equation}\mspace{14mu} 3} \right)\end{matrix}$where FB represents the feedback signal 140, V_(D) represents a forwardvoltage of the diode 410, and R₁ represents the resistance of theresistor 414. In addition, R₂ represents the resistance of the resistor412, and ΔV_(slope) represents a parameter associated with the dutycycle of the signal 148 (e.g., a PWM signal). For example, the signal454 is determined according to the following equation:

$\begin{matrix}{{FB}_{div} = {\frac{{FB} - V_{D}}{R_{1} + R_{2}} \times R_{1}}} & \left( {{Equation}\mspace{14mu} 4} \right)\end{matrix}$where FB_(div) represents the signal 454. In another example, if thecurrent-sensing signal 142 becomes equal to or larger than the signal460 (e.g., V_(th_pwm)) in magnitude, the comparator 428 outputs thesignal 474 to change the signal 148 (e.g., PWM) in order to open (e.g.,turn off) the switch 104.

According to yet another embodiment, if the output load increases, thefeedback signal 140 increases in magnitude, and the signal 460 (e.g.,V_(th_pwm)) increases in magnitude. For example, if the signal 460(e.g., V_(th_pwm)) is equal to the first threshold signal 462 (e.g.,V_(p_1)) in magnitude and the current-sensing signal 142 becomes equalto or larger than the first threshold signal 462 (e.g., V_(p_1)), thecomparator 422 outputs the signal 468 (e.g., V₁) at a logic high levelin order to trigger the counter component 436 to count time. In anotherexample, the flip-flop component 432 samples the signal 468 upon afalling edge of the signal 148. In yet another example, if the signal468 is kept at the logic high level for a first predetermined timeperiod (e.g., T₁), the counter component 436 outputs a signal 476 (e.g.,at a logic high level) to trigger the first-level protection. In yetanother example, if the first-level protection is triggered, the powerconversion system 100 is shut down and the switch 104 keeps open. In yetanother example, after being shut down, the system 100 restarts (e.g.,automatically or manually) and starts modulation again.

According to yet another embodiment, if the output load continues toincrease, the feedback signal 140 continues to increase in magnitude,and the signal 460 (e.g., V_(th_pwm)) increases in magnitude. Forexample, if the signal 460 (e.g., V_(th_pwm)) becomes equal to or largerthan the second threshold signal 464 (e.g., V_(p_2)) in magnitude, themaximum threshold of the current-sensing signal 142 is clamped at thesecond threshold signal 464 (e.g., V_(p_2)). In another example, if thecurrent-sensing signal 142 becomes equal to or larger than the secondthreshold signal 464 (e.g., V_(p_2)) in magnitude, the comparator 426outputs the signal 472 (e.g., at the logic high level) to change thesignal 148 in order to open (e.g., turn off) the switch 104. In yetanother example, if the signal 454 (e.g., FB_div) becomes equal to orlarger than the signal 494 (e.g., V_(th_pk)) in magnitude, thecomparator 424 outputs the signal 470 (e.g., V₂) at a logic high levelin order to trigger the counter component 438 to count time. In yetanother example, if the signal 470 is kept at the logic high level for asecond predetermined time period (e.g., T₂), the counter component 438outputs a signal 478 (e.g., at a logic high level) to trigger asecond-level protection. In yet another example, if the second-levelprotection is triggered, the power conversion system 100 is shut downand the switch 104 keeps open. In yet another example, after being shutdown, the system 100 restarts (e.g., automatically or manually) andstarts modulation again. In yet another example, the secondpredetermined time period (e.g., T₂) is smaller in duration than thefirst predetermined time period (e.g., T₁).

As discussed above and further emphasized here, FIG. 3(b) is merely anexample, which should not unduly limit the scope of the claims. One ofordinary skill in the art would recognize many variations, alternatives,and modifications. According to certain embodiments, the modulationcomponent 120 may include components not shown in FIG. 3(b). Accordingto some embodiments, one or more components included in the modulationcomponent 120, as shown in FIG. 3(b), may be omitted. According tocertain embodiments, the logic controller 122 may include components notshown in FIG. 3(b). According to some embodiments, one or morecomponents included in the logic controller 122, as shown in FIG. 3(b),may be omitted.

FIGS. 4(a) and 4(b) are simplified diagrams showing the peak value ofthe current-sensing signal 142 and the modulation frequency of the powerconversion system 100 as functions of the feedback signal 140respectively according to certain embodiments of the present invention.These diagrams are merely examples, which should not unduly limit thescope of the claims. One of ordinary skill in the art would recognizemany variations, alternatives, and modifications. The waveform 502represents the feedback signal 140 as a function of the peak values ofthe current-sensing signal 142. The waveform 504 represents the feedbacksignal 140 as a function of the modulation frequency of the powerconversion system 100. For example, the modulation frequency of thepower conversion system 100 is the same as the modulation frequencyassociated with the signal 148 and the signal 196.

As shown in FIG. 4(a) and FIG. 4(b), the power conversion system 100implements two-level protection, in some embodiments. For example, ifthe feedback signal 140 is in a range between a magnitude FB0 andanother magnitude FBb, the peak value of current-sensing signal 142 hasa minimum magnitude 506 (e.g., V_(cs_min)), as shown by the waveform502. In another example, if the feedback signal 140 is in a rangebetween the magnitude FBb and a magnitude FB1, the peak value of thecurrent-sensing signal 142 increases (e.g., linearly or non-linearly) inmagnitude with the feedback signal 140 (e.g., from V_(cs_min) at FBb toV_(p_1) at FB1), as shown by the waveform 502. In yet another example,if the peak value of the current-sensing signal 142 reaches the firstthreshold magnitude 510 (e.g., V_(p_1) at FB1), a first level protectionis triggered. In yet another example, if the feedback signal 140 is in arange between the magnitude FB1 and the magnitude FB4, the peak value ofthe current-sensing signal 142 increases (e.g., linearly ornon-linearly) in magnitude with the feedback signal 140 (e.g., fromV_(p_1) at FB1 to V_(p_2) at FB4), as shown by the waveform 502. In yetanother example, if the peak value of the current-sensing signal 142reaches the maximum magnitude 508 (e.g., V_(p_2)), a second levelprotection is triggered. In yet another example, the minimum magnitude506 (e.g., V_(cs_min)) is one fifth of the first threshold magnitude 510(e.g., V_(p_1)). In yet another example, the minimum magnitude 506(e.g., V_(cs_min)) is one third of the first threshold magnitude 510(e.g., V_(p_1)).

In one embodiment, the modulation frequency keeps at a particularmagnitude (e.g., F_(burst)) which indicates a burst mode, if thefeedback signal 140 is within a range between the magnitude FB0 and themagnitude FBa, as shown by the waveform 504. For example, if thefeedback signal 140 is in a range between the magnitude FBa and amagnitude FBc, the modulation frequency increases (e.g., linearly ornon-linearly) in magnitude with the feedback signal 140 (e.g., fromF_(burst) at FBa to F_(p_1) at FBc), as shown by the waveform 504. Inanother example, the first threshold value 512 (e.g., F_(p_1))corresponds to the first level protection. In yet another example, ifthe feedback signal 140 is in a range between the magnitude FBc and amagnitude FB2, the modulation frequency keeps at the first thresholdvalue F_(p_1), as shown by the waveform 504. In yet another example, ifthe feedback signal 140 is equal to the magnitude FB2, the system 100operates normally under a full-load condition. According to someembodiments, the full-load condition represents a maximum load conditionunder which the system 100 can deliver the required power to the outputload continuously for an extended period of time. For example, themodulation frequency increases (e.g., linearly or non-linearly) from thefirst threshold value 512 (e.g., F_(p_1)) to a second threshold value514 (e.g., F_(p_2)) if the feedback signal 140 increases (e.g., linearlyor non-linearly) from the magnitude FB2 to the magnitude FB3. In anotherexample, the modulation frequency keeps at the second threshold value514 (e.g., F_(p_2)), if the feedback signal 140 becomes equal to orlarger than the magnitude FB3. In yet another example, the secondthreshold value (e.g., F_(p_2)) corresponds to the second levelprotection. In yet another example, if the feedback signal 140 is equalto the magnitude FB3, the output load reaches a peak value. In yetanother example, a peak magnitude (e.g., FB_PK) of the feedback signal140 corresponds to a peak-load condition. According to certainembodiments, the peak-load condition represents a load condition underwhich a peak power, much larger than that under the full-load condition,can be delivered during only a short period of time (e.g., 10 ms). Forexample, FB0≤FBa≤FBb≤FBc≤FB1≤FB2≤FB3≤FB4≤FB_PK.

Referring to FIG. 3(a), the counter 336 associated with the first levelprotection is triggered to count time if the current-sensing signal 142becomes equal to or larger than the first threshold signal 362 (e.g.,V_(p_1)) in magnitude according to some embodiments. For example, asshown in FIG. 4(a), the first threshold signal 362 (e.g., V_(p_1))corresponds to the magnitude FB1 of the feedback signal 140. In anotherexample, the counter 336 associated with the first level protection istriggered to count time if the feedback signal 140 is equal to or largerthan the magnitude FB1. According to certain embodiments, as shown inFIG. 3(a), the counter 338 associated with the second level protectionis triggered to count time if the current-sensing signal 142 becomesequal to or larger than the second threshold signal 364 (e.g., V_(p_2))in magnitude. For example, as shown in FIG. 4(a), the second thresholdsignal 364 (e.g., V_(p_2)) corresponds to the magnitude FB4 of thefeedback signal 140. In another example, the counter 338 associated withthe second level protection is triggered to count time if the feedbacksignal 140 is equal to or larger than the magnitude FB4.

Referring to FIG. 3(b), the counter 436 associated with the first levelprotection is triggered to count time if the current-sensing signal 142becomes equal to or larger than the first threshold signal 462 (e.g.,V_(p_1)) in magnitude according to some embodiments. For example, asshown in FIG. 4(a), the first threshold signal 462 (e.g., V_(p_1))corresponds to the magnitude FB1 of the feedback signal 140. In anotherexample, the counter 436 associated with the first level protection istriggered to count time if the feedback signal 140 is equal to or largerthan the magnitude FB1. According to certain embodiments, as shown inFIG. 3(b), the counter 438 associated with the second level protectionis triggered to count time if the signal 454 associated with thefeedback signal 140 becomes equal to or larger than the threshold signal494 (e.g., V_(th_PK)) in magnitude. For example, as shown in FIG. 4(b),the threshold signal 494 (e.g., V_(th_PK)) corresponds to the magnitudeFB_PK of the feedback signal 140. In another example, the counter 438associated with the second level protection is triggered to count timeif the feedback signal 140 is equal to or larger than the magnitudeFB_PK.

Referring back to FIG. 1, the power conversion system 100 operates in aconstant-voltage mode, in some embodiments. For example, the outputvoltage 198 is regulated to be at a desired level based on at leastinformation associated with the feedback signal 140. In another example,if the output voltage 198 deviates from the desired level by a smallamount, the feedback signal 140 has a significant change, and thus thecontroller 102 changes at least the signal 196 in order to adjust theoutput voltage 198 back to the desired level.

If the output voltage 198 of the power conversion system 100 remainsapproximately constant, the simplified timing diagram for the outputpower as shown in FIG. 2 approximates the timing diagram for the outputcurrent of the power conversion system 100, according to certainembodiments. For example, a two-level over-current protection may beimplemented for the power conversion system 100, where the outputcurrent of the power conversion system 100 is subject to two currentthresholds. In another example, if the output current is below a firstcurrent threshold, the power conversion system 100 performs normaloperations. In yet another example, if the output current becomes equalto or larger than a first current threshold and remains equal to orlarger than the first current threshold for at least a firstpredetermined time period, the two-level protection component 126changes the signal 146 in order to shut down the power conversion system100. In yet another example, if the output current becomes equal to orlarger than a second current threshold (e.g., larger than the firstcurrent threshold) and also remains equal to or larger than the secondpredetermined time period, the two-level protection component 126changes the signal 146 in order to shut down the power conversion system100. In yet another example, the second predetermined period of time issmaller than the first predetermined period of time in duration.

According to another embodiment, a system controller for protecting apower conversion system includes a two-level protection component and adriving component. The two-level protection component is configured todetect an output power of a power conversion system and generate aprotection signal based on at least information associated with theoutput power of the power conversion system. The driving component isconfigured to generate a drive signal based on at least informationassociated with the protection signal and output the drive signal to aswitch associated with a primary current flowing through a primarywinding of the power conversion system. The driving component is furtherconfigured to, generate the drive signal corresponding to a firstswitching frequency to generate the output power equal to a first powerthreshold, and generate the drive signal corresponding to a secondswitching frequency to generate the output power equal to a second powerthreshold, the second power threshold being larger than the first powerthreshold, the second switching frequency being larger than the firstswitching frequency. The two-level protection component is furtherconfigured to, in response to the output power being equal to or largerthan the first power threshold for a first predetermined period of time,generate the protection signal to cause the switch to open and remainopen to protect the power conversion system, and in response to theoutput power being equal to or larger than the second power thresholdfor a second predetermined period of time, generate the protectionsignal to cause the switch to open and remain open to protect the powerconversion system, the second predetermined period of time being shorterthan the first predetermined period of time. For example, the systemcontroller is implemented according to FIG. 1, FIG. 2, FIG. 3(a), FIG.3(b), FIG. 4(a), and/or FIG. 4(b).

According to yet another embodiment, a system controller for regulatinga power conversion system includes a modulation component and a drivingcomponent. The modulation component is configured to receive a feedbacksignal associated with an output signal of a power conversion system andgenerate a modulation signal based on at least information associatedwith the feedback signal, the modulation signal being associated with amodulation frequency. The driving component is configured to generate adrive signal based on at least information associated with themodulation signal and output the drive signal to a switch associatedwith a primary current flowing through a primary winding of the powerconversion system, the drive signal being associated with the modulationfrequency. The modulation component is further configured to, increasethe modulation frequency from a first frequency magnitude to a secondfrequency magnitude in response to the feedback signal increasing from afirst feedback magnitude to a second feedback magnitude, keep themodulation frequency at the second frequency magnitude if the feedbacksignal is larger than the second feedback magnitude and smaller than thethird feedback magnitude, and increase the modulation frequency from thesecond frequency magnitude to a third frequency magnitude in response tothe feedback signal increasing from the third feedback magnitude to afourth feedback magnitude. For example, the system controller isimplemented according to FIG. 1, FIG. 2, FIG. 3(a), FIG. 3(b), FIG.4(a), and/or FIG. 4(b).

In one embodiment, a method for protecting a power conversion systemincludes, detecting an output power of a power conversion system,generating a protection signal based on at least information associatedwith the output power of the power conversion system, and receiving theprotection signal. The method further includes, processing informationassociated with the protection signal, generating a drive signal basedon at least information associated with the protection signal, andoutputting the drive signal to a switch associated with a primarycurrent flowing through a primary winding of the power conversionsystem. The generating a drive signal based on at least informationassociated with the protection signal includes, generating the drivesignal corresponding to a first switching frequency to generate theoutput power equal to a first power threshold, and generating the drivesignal corresponding to a second switching frequency to generate theoutput power equal to a second power threshold, the second powerthreshold being larger than the first power threshold, the secondswitching frequency being larger than the first switching frequency. Thegenerating a protection signal based on at least information associatedwith the output power of the power conversion system includes, inresponse to the output power being equal to or larger than the firstpower threshold for a first predetermined period of time, generating theprotection signal to cause the switch to open and remain open to protectthe power conversion system, and in response to the output power beingequal to or larger than the second power threshold for a secondpredetermined period of time, generating the protection signal to causethe switch to open and remain open to protect the power conversionsystem, the second predetermined period of time being shorter than thefirst predetermined period of time. For example, the method isimplemented according to FIG. 1, FIG. 2, FIG. 3(a), FIG. 3(b), FIG.4(a), and/or FIG. 4(b).

In another embodiment, a method for regulating a power conversion systemincludes, receiving a feedback signal associated with an output signalof a power conversion system, processing information associated with thefeedback signal, and generating a modulation signal based on at leastinformation associated with the feedback signal, the modulation signalbeing associated with a modulation frequency. The method furtherincludes, receiving the modulation signal, processing informationassociated with the modulation signal, generating a drive signal basedon at least information associated with the modulation signal, andoutputting the drive signal to a switch associated with a primarycurrent flowing through a primary winding of the power conversionsystem, the drive signal being related to the modulation frequency. Thegenerating a modulation signal based on at least information associatedwith the feedback signal includes, increasing the modulation frequencyfrom a first frequency magnitude to a second frequency magnitude inresponse to the feedback signal increasing from a first feedbackmagnitude to a second feedback magnitude, keeping the modulationfrequency at the second frequency magnitude if the feedback signal islarger than the second feedback magnitude and smaller than the thirdfeedback magnitude, and increasing the modulation frequency from thesecond frequency magnitude to a third frequency magnitude in response tothe feedback signal increasing from the third feedback magnitude to afourth feedback magnitude. For example, the method is implementedaccording to FIG. 1, FIG. 2, FIG. 3(a), FIG. 3(b), FIG. 4(a), and/orFIG. 4(b).

For example, some or all components of various embodiments of thepresent invention each are, individually and/or in combination with atleast another component, implemented using one or more softwarecomponents, one or more hardware components, and/or one or morecombinations of software and hardware components. In another example,some or all components of various embodiments of the present inventioneach are, individually and/or in combination with at least anothercomponent, implemented in one or more circuits, such as one or moreanalog circuits and/or one or more digital circuits. In yet anotherexample, various embodiments and/or examples of the present inventioncan be combined.

Although specific embodiments of the present invention have beendescribed, it will be understood by those of skill in the art that thereare other embodiments that are equivalent to the described embodiments.Accordingly, it is to be understood that the invention is not to belimited by the specific illustrated embodiments, but only by the scopeof the appended claims.

What is claimed is:
 1. A system controller for protecting a power conversion system, the system controller comprising: a protection signal generator configured to detect an output power of a power conversion system and generate a protection signal; and a drive signal generator configured to generate a drive signal and output the drive signal to a switch; wherein the drive signal generator is further configured to: generate the drive signal corresponding to a first switching frequency to generate the output power equal to a first power threshold; and generate the drive signal corresponding to a second switching frequency to generate the output power equal to a second power threshold, the second power threshold being larger than the first power threshold, the second switching frequency being larger than the first switching frequency; wherein the protection signal generator is further configured to: in response to the output power being larger than the first power threshold for a first predetermined period of time, generate the protection signal to cause the switch to open and remain open to protect the power conversion system; and in response to the output power being larger than the second power threshold for a second predetermined period of time, generate the protection signal to cause the switch to open and remain open to protect the power conversion system, the second predetermined period of time being shorter than the first predetermined period of time.
 2. The system controller of claim 1 wherein the protection signal generator is further configured to detect an output current indicating the output power of the power conversion system and generate the protection signal based on at least information associated with the output current.
 3. The system controller of claim 1 wherein the protection signal generator is further configured to, in response to the output power being larger than the first power threshold for the first predetermined period of time, generate the protection signal to cause the system controller to be shut down; and in response to the output power being larger than the second power threshold for the second predetermined period of time, generate the protection signal to cause the system controller to be shut down.
 4. The system controller of claim 1 wherein the protection signal generator is further configured to, in response to the output power being larger than the first power threshold for the first predetermined period of time, generate the protection signal to cause the switch to be opened without any modulation; and in response to the output power being larger than the second power threshold for the second predetermined period of time, generate the protection signal to cause the switch to be opened without any modulation.
 5. The system controller of claim 1 wherein the protection signal generator is further configured to, in response to the output power being larger than the first power threshold for the first predetermined period of time, generate the protection signal to cause the switch to remain open for a first time duration longer than at least a first switching period corresponding to the first switching frequency; and in response to the output power being larger than the second power threshold for the second predetermined period of time, generate the protection signal to cause the switch to remain open for a second time duration longer than at least a second switching period corresponding to the second switching frequency.
 6. The system controller of claim 1, and further comprising: a modulation signal generator configured to receive a current-sensing signal and a feedback signal and generate a modulation signal based on at least information associated with the current-sensing signal and the feedback signal; wherein the drive signal generator is further configured to generate the drive signal based on at least information associated with the modulation signal.
 7. The system controller of claim 1 wherein the protection signal generator includes: a first comparator configured to receive a current-sensing signal and a first threshold signal and generate a first comparison signal based at least in part on the current-sensing signal and the first threshold signal, the first threshold signal corresponding to the first power threshold; and a second comparator configured to receive the current-sensing signal and a second threshold signal and generate a second comparison signal based at least in part on the current-sensing signal and the second threshold signal, the second threshold signal corresponding to the second power threshold.
 8. The system controller of claim 1 wherein the protection signal generator includes: a first comparator configured to receive a current-sensing signal and a first threshold signal and generate a first comparison signal based at least in part on the current-sensing signal and the first threshold signal, the first threshold signal corresponding to the first power threshold; and a second comparator configured to receive a feedback signal and a second threshold signal and generate a second comparison signal based at least in part on the feedback signal and the second threshold signal, the second threshold signal corresponding to the second power threshold.
 9. A system controller for regulating a power conversion system, the system controller comprising: a modulation signal generator configured to receive a feedback signal associated with the power conversion system and generate a modulation signal based on at least information associated with the feedback signal, the modulation signal being associated with a modulation frequency; a drive signal generator configured to generate a drive signal based on at least information associated with the modulation signal and output the drive signal to a switch, the drive signal being associated with the modulation frequency; and a protection signal generator; wherein the modulation signal generator is further configured to: increase the modulation frequency from a first frequency magnitude to a second frequency magnitude in response to the feedback signal increasing from a first feedback magnitude to a second feedback magnitude; and increase the modulation frequency from the second frequency magnitude to a third frequency magnitude in response to the feedback signal increasing from a third feedback magnitude to a fourth feedback magnitude, the third feedback magnitude being larger than the second feedback magnitude; wherein the protection signal generator is configured to: in response to determining that a first level protection or a second level protection is triggered, generate a protection signal to cause the switch to open and remain open to protect the power conversion system; wherein: the second frequency magnitude corresponds to the first level protection; and the third frequency magnitude corresponds to the second level protection.
 10. The system controller of claim 9, and further comprising a logic controller configured to generate a first signal based on at least information associated with the modulation signal and output the first signal to the drive signal generator.
 11. The system controller of claim 9 wherein the modulation signal generator is further configured to increase the modulation frequency linearly from the first frequency magnitude to the second frequency magnitude in response to the feedback signal increasing from the first feedback magnitude to the second feedback magnitude.
 12. The system controller of claim 11 wherein the first frequency magnitude corresponds to a burst mode of the power conversion system.
 13. The system controller of claim 11 wherein the modulation signal generator is further configured to increase the modulation frequency linearly from the second frequency magnitude to the third frequency magnitude in response to the feedback signal increasing from the third feedback magnitude to the fourth feedback magnitude.
 14. The system controller of claim 13 wherein the third feedback magnitude corresponds to a full-load condition of the power conversion system.
 15. A method for protecting a power conversion system, the method comprising: detecting an output power of a power conversion system; generating a protection signal; processing information associated with the protection signal; generating a drive signal; and outputting the drive signal to a switch; wherein the generating a drive signal includes: generating the drive signal corresponding to a first switching frequency to generate the output power equal to a first power threshold; and generating the drive signal corresponding to a second switching frequency to generate the output power equal to a second power threshold, the second power threshold being larger than the first power threshold, the second switching frequency being larger than the first switching frequency; wherein the generating a protection signal includes: in response to the output power being larger than the first power threshold for a first predetermined period of time, generating the protection signal to cause the switch to open and remain open to protect the power conversion system; and in response to the output power being larger than the second power threshold for a second predetermined period of time, generating the protection signal to cause the switch to open and remain open to protect the power conversion system, the second predetermined period of time being shorter than the first predetermined period of time.
 16. A method for regulating a power conversion system, the method comprising: receiving a feedback signal associated with the power conversion system; generating a modulation signal based on at least information associated with the feedback signal, the modulation signal being associated with a modulation frequency; processing information associated with the modulation signal; generating a drive signal based on at least information associated with the modulation signal; outputting the drive signal to a switch, the drive signal being associated with the modulation frequency; and in response to determining that a first level protection or a second level protection is triggered, generating a protection signal to cause the switch to open and remain open to protect the power conversion system; wherein the generating a modulation signal based on at least information associated with the feedback signal includes: increasing the modulation frequency from a first frequency magnitude to a second frequency magnitude in response to the feedback signal increasing from a first feedback magnitude to a second feedback magnitude; and increasing the modulation frequency from the second frequency magnitude to a third frequency magnitude in response to the feedback signal increasing from a third feedback magnitude to a fourth feedback magnitude, the third feedback magnitude being larger than the second feedback magnitude; wherein: the second frequency magnitude corresponds to the first level protection; and the third frequency magnitude corresponds to the second level protection. 